This invention relates to an electron emission element of the field emission type suitable for use for a microwave vacuum tube, a light source, an amplification element, a high-speed switching element, a sensor or the like and an image display device using such an electron emission element as a cathode, and more particularly to an electron emission element conveniently applicable to a color display device in which a field emission cathode is incorporated.
Application of an electric field of about 10.sup.9 V/m to a metal surface or a semiconductor surface causes electrons to pass through a barrier by a tunnel effect, resulting in the electrons being discharged into a vacuum atmosphere even at a normal temperature. Such a phenomenon is called field emission and a cathode constructed so as to emit electrons based on such a principle is called a field emission cathode (hereinafter also referred to as "FEC").
Recent progress in techniques for processing a semiconductor has led to development of an FEC of the surface-emission type including FEC arrays of a micron size.
Now, a conventional image display device will be described by way of example with reference to FIG. 28.
The conventional image display device includes an anode substrate 400 and a cathode substrate 401 arranged oppositely to each other and side plates (not shown) arranged so as to surround an outer periphery of each of the substrates 400 and 401, resulting in an envelope 402 being provided, which is then evacuated to a high vacuum. The anode substrate 400 is provided on an inner surface thereof with a display section 405 comprising a light-permeable anode conductor 403 and phosphor layers 404 deposited thereon.
The cathode substrate 401 is provided on an inner surface thereof opposite to the display section 405 of the anode substrate 400 with field emission elements each including emitter electrodes 406 of a conical shape. More particularly, the cathode substrate 401 is provided on the inner surface thereof with stripe-like cathode conductors 407, each of which is provided thereon with an insulating layer 409 formed with openings 408. The apertures 408 each are provided therein with an emitter electrode 406 acting as a conical electron emitter while being arranged on the cathode conductor 407. The insulating layer 409 is provided on an upper surface thereof with stripe-like gate electrodes 411 each formed with apertures 410 in a manner to be aligned with the openings 408 and arranged so as to extend in a direction across the cathode conductors 407.
Thus, the cathode conductors 407 and gate electrodes 411 cooperate with each other to form a matrix, so that when application of an anode voltage of a predetermined level to the display section 405 and driving of each of the cathode conductors 407 and gate electrodes 411 at a suitable timing permit the emitter electrodes 406 from which electrons are to be emitted to be selected, resulting in the phosphor layers 404 of the display section 405 opposite to the selected emitter electrodes 406 being selectively driven, leading to a desired luminous display.
In the conventional image display device, the emitter electrodes 406 each are formed into a conical shape and the gate electrodes 411 each are arranged so as to surround a tip end of each of the emitter electrodes 406, so that electrons emitted from the emitter electrodes 406 are caused to be spread at an angle of, for example, about 30 degrees on each of sides although it is of course that the spreading is somewhat varied depending on a voltage applied to the anode conductor 403 of the display section 405. Thus, when it is desired to obtain a display with high definition and fine picture cell pitches, it is required to decrease an interval between the field emission type cathodes and the display section or reduce a region in which the field emission type cathodes are arranged with respect to pitches of arrangement of picture cells. Otherwise, electrons diffused as described above are caused to impinge on adjacent picture cells, leading to leakage luminescence of the adjacent picture cells.
Unfortunately, in order to increase luminance of picture cells, it is required to increase an anode voltage of the display section. However, this requires to somewhat increase a distance between the display device and the cathodes, to thereby ensure insulation therebetween. For example, supposing that an anode voltage of 1 kV is applied to the display section 405, it is required that the interval between the phosphor layers 404 and the gate electrodes 411 is set to be 150 to 200 um.
Such an electron emission element of the field emission type as described above is widely used in the field of a microwave vacuum tube, a light source, an amplification element, a high-speed switching element, a sensor or the like.
Now, a display with high definition which is obtained at picture cell pitches as small as, for example, 0.33 mm or less in a display device of which a display is selected by means of gate electrodes and emitter electrodes will be discussed hereinafter. When it is desired to obtain such a display in the form of, for example, a full color display, it is required to construct a color display device in such a manner that picture cells which are constituted by phosphor layers R, G and B are formed into a width of about 80 um and arranged at intervals of about 20 um.
Supposing that a distance between the phosphor layers 404 and the gate electrodes 411 is set to be 150 um and an angle of spreading of electrons emitted by the conical emitter electrodes 406 is 30 degrees on each side, the electrons are caused to spread by a distance of about 80 um on each side. This leads to a failure in proper operation of a display device including picture cells having such dimensions as described above, to thereby fail to provide the display with such high definition as described above. On the contrary, a decrease in distance between the phosphor layers 404 and the gate electrodes 411 to about 50 um permits spreading of the electrons to be minimized. However, this renders an increase in anode voltage impossible in view of dielectric strength, leading to a decrease in luminance.
Also, when spreading of electrons emitted occurs in a microwave vacuum tube, a light source, an amplification element, a high-speed switching element, a sensor or the like in which the above-described field emission element is incorporated, the amount of electrons reaching an anode (collector) is reduced, leading to a deterioration in S/N ratio with respect to input power or a failure to increase sensitivity, to thereby render light emission unstable.
A field emission cathode (FEC) is generally formed into a planar configuration, so that a field emission cathode of the surface emission type may be provided. Thus, application of such a field emission cathode of the surface emission type to a color display device has been proposed. Such a conventional color display device may be generally constructed in such a manner as shown in FIG. 23.
More particularly, a second substrate 105 arranged opposite to a first substrate 101 is provided thereon with a plurality of anode electrode groups each including three stripe-like anode electrode elements 106-1, 106-2 and 106-3 which are provided thereon with a phosphor (R) of a red luminous color, a phosphor (G) of a green luminous color and a phosphor (B) of a blue luminous color, respectively. The stripe-like anode electrode elements 106-1, 106-2 and 106-3 for the phosphors of the respective luminous colors are commonly connected to anode lead-out electrodes A1, A2 and A3, respectively, which are then led out of the second substrate 105.
The first substrate 101 is formed thereon with cathode electrodes 102, each of which is formed thereon with emitter arrays 104 including a plurality of conical emitters for field-emitting electrons. Each of the cathode electrodes 102 is formed thereon with gate electrodes 103 while keeping the gate electrodes 103 insulated from the cathode electrode 102.
Thus, the anode electrode elements 106-1 cooperate with each other to form an anode electrode for emitting only light of a red luminous color, the anode electrode elements 106-2 form an anode electrode for emitting only light of a green luminous color and the anode electrode elements 106-3 form an anode electrode for emitting only light of a blur luminous color.
Arrangement of the above-described electrodes is shown in FIG. 22, which is viewed from a side of the anode electrodes. As shown in FIG. 22, the anode lead-out electrodes A1, A2 and A3 are led out of the anode electrode elements 106-1, 106-2 and 106-3 on both sides of the substrate, respectively. The gate electrodes 103 (103-1, 103-2, . . . , 103-1) are formed in a manner to be spaced from and parallel to the anode electrode elements 106-1 to 106-3. The gate electrodes 103 each are provided with gate lead-out electrodes GT1, GT2, . . . , GTl in a manner to be led out thereof, respectively.
In order that the emitter arrays 104 (104-1, 104-2, . . . ) from which emission of electrons is controlled by the gate electrodes 103-1, 103-2, . . . , 103-1 cause each one set of picture cells R, G and B to emit light, the gate electrodes 103-1, 103-2, 103-1 are formed so as to straddle the anode electrode elements 106-1 to 106-3 corresponding to each one set of picture cells R, G and B.
The cathode electrodes 102 are formed into a stripe-like shape and arranged below the gate electrodes 103-1, 103-2, . . . , 103-1 so as to extend in a direction perpendicular to the anode electrodes 106-1 to 106-3. Also, the cathode electrodes 102 are provided with cathode lead-out electrode C1, C2, . . . , Cn in a manner to be led out thereof, respectively. The emitter arrays 104 are arranged on each of the cathode electrodes 102. The anode electrode elements 106-1 each have the phosphor R of a red luminous color deposited thereon, the anode electrode elements 106-2 each have the phosphor G of a green luminous color deposited thereon, and the anode electrode elements 106-3 each have the phosphor B of a blue luminous color deposited thereon.
In order to cause the color display device constructed as described above to display a color image, a contact al to which the lead-out electrode A1 of each of the anode electrode elements 106-1 is connected is selected through changing-over of a switch 100 to cause an anode voltage Ea to be applied to the anode electrode elements 106-1. Concurrently, selection of the cathode electrodes 102 is carried out by closing a switch 110 and color data on a red luminous color are fed to the gate lead-out electrodes GT1, GT2, . . . , GTl to cause the display device to display an image of a red luminous color on one line. Then, the cathode lead-out electrodes C1 to Cn are scanned in turn to cause the display device to display an image of a red luminous color.
Subsequently, the switch 100 is changed over to a position of a contact a2 to which the anode lead-out electrode A2 is connected, to thereby cause the anode voltage Ea to be applied to the anode electrode elements 106-2. Concurrently, the cathode lead-out electrodes C1 to Cn are scanned in turn and data on a green luminous color are fed to the gate electrodes GT1 to GTl in synchronism with the scanning, resulting in the display device displaying an image of a green luminous color.
Thereafter, the switch 100 is changed over to a position of a contact a3 to which the anode lead-out electrode A3 is connected to cause the anode voltage Ea to be applied to the anode electrode elements 106-3. Concurrently, the cathode lead-out electrodes C1 to Cn are scanned in turn and data on a blue luminous color are fed to the gate electrodes GT1 to GTl in synchronism with the canning, to thereby cause the display device to display an image of a blue luminous color. Thus, the conventional color display device displays a color image according to a surface sequential system.
In the conventional color display device in which the anode electrodes each are constituted by the three anode electrode elements, it is required to lead out each three anode lead-out electrodes A1, A2 and A3 from the second substrate 105 because the anode electrode elements 106-1, 106-2 and 106-3 are formed on the second substrate 105. Unfortunately, leading-out of each three anode lead-out electrodes A1, A2 and A3 from the second substrate causes multi-level crossing of the electrodes as indicated at reference characters a, b and c in FIG. 22, so that it is required to arrange the electrodes in a three-dimensional wiring manner.
Also, each of the anode electrodes is formed of three anode electrode elements, therefore, a duty determined by the number of times of scanning of the cathode lead-out electrodes C1 to Cn is reduced to a level of 1/3, resulting in an image plane of the display device being decreased in brightness.
In order to solve the above-described problems, it would be considered that a construction of the color display device in such a manner that only one anode lead-out electrode is arranged and the cathode lead-out electrodes and gate lead-out electrodes are scanned to selectively drive the phosphors R, G and B arranged on the anode electrode elements permits the display device to display a color image while eliminating the above-described three-dimensional arrangement or wiring of the anode lead-out electrodes. However, such a construction of the display device fails to prevent bleeding of an image due to leakage luminescence of adjacent phosphors because electrons emitted from the cathode electrodes reach the anode electrodes while spreading to a degree, as will be understood from the widely-known fact that electrons emitted from cathode electrodes generally travel to anodes while spreading at an angle of about 30 degrees.
Also, in the conventional image display device in which the field emission cathodes are incorporated, a voltage applied to the anode electrodes is within a range between hundreds volts and thousands bolts, to thereby fail to meet all luminous characteristics of the phosphors such as luminous efficiency, color purity, durability and the like. Thus, when it is desired to directly observe luminescence from the phosphors, the image display device fails to permit the phosphors to exhibit desired luminance. In particular, the conventional image display device causes the phosphor of a red luminous color to be deteriorated in luminous characteristics or efficiency as compared with the phosphors of other luminous colors.
Further, the conventional image display device uses a filter in order to increase color purity of a display and contrast thereof to obtain a plurality of luminous colors from the same phosphor. Unfortunately use of the filter substantially fails to permit the phosphors to exhibit desired luminescence and luminance and causes non-uniformity in luminescence and luminance between the phosphors.